To make the cake, use a whisk or an electric beater to whisk together the eggs, sugar, bicarbonate of soda and honey until light and fluffy.
Melt the butter in a saucepan over very low heat. When it has just melted, add the egg mixture and stir with a wooden spoon or spatula until the butter is incorporated. Add the flour, in batches, stirring well after each addition until it is well combined. Remove the pan from the heat and allow to cool to room temperature.
Preheat the oven to 160°C (320°F/Gas 2–3).
Rolling the cake, which is the next step, can be a bit tricky — I recommend taking a square of baking paper and putting a generous amount of flour on it. Then take a ¼ cup measure and dust the inside with flour (if you fill it and dump it out again this will prevent the mixture from sticking to the cup as you measure it). Scoop a generous ¼ cup of the cake mixture and dump it on the baking paper right in the middle of all that flour. Roll it around until it is thoroughly coated, then press it out into a disc. Dust the top of the disc and coat a rolling pin with flour, then carefully roll it out into a very thin round disc, about 2 mm thick. Don’t worry, it will puff up when you cook it.
Carefully shake off any excess flour then place it on a tray and cook in the oven for 2–4 minutes — it should be deep golden all over. Remove from the oven and let it cool on a wire rack. You can usually roll out the next cake disc while the first is cooking. Repeat this process to make 5 or 6 discs. Use a side plate to cut each disc into neat, even-sized circles.
While the discs are cooling, make the filling. Put the sour cream, honey and sugar into a mixing bowl and whisk by hand or use an electric beater to whisk until it is thick and the sugar has completely dissolved.
Place one cake disc on a sheet of baking paper, then spread about one-sixth of the filling evenly over the top. Place a cake disc over the top, followed by another layer of filling. Repeat until you have used up all the discs and all but one-sixth of the filling (if you made more discs then adjust accordingly). Spread the remaining sour cream mix around the edge of the cake. Finally, press the crushed walnuts onto the top and sides of the cake. Place the whole thing in a container large enough to hold the cake and cover to seal, then refrigerate until it is set and thoroughly cooled, at least 4 hours.